This is a pop-science description of the Big Bang model. The standard cosmological model of a hot Big Bang is nearly universally-accepted, on extremely strong empirical/observational footing... and doesn't include anything about a "beginning of the universe".
The hot Big Bang model merely posits a hot, dense early state of the universe some 13.8 billion years ago, from which it has since been expanding + cooling, leading to the present state of the universe which we observe. And the primary pieces of evidence for this are:
- the observation that space is expanding
- the observation of nearly uniformly-distributed leftover heat from this early state, the cosmic microwave background radiation
- the observation of the relative abundances of elements predicted by the big bang model
The tl;dr version is that we observe that the universe is expanding, and has this leftover heat glow. If the universe is presently expanding and cooling, then it follows that at some prior time it was denser and warmer. You rewind the cosmic clock backwards as far as you can go, and you're left with a very hot, very dense early state. This is the Big Bang; a hot dense early state of the universe, almost 14 billion years ago.
But we can only rewind the clock back so far, before our theory (general relativity) breaks down and ceases to be a good description of physical reality- at some point, quantum effects become significant, and general relativity is not a quantum theory. But we don't have a good alternative, because we do not currently have an accepted theory of quantum gravity (there are a variety of candidates, like string/M-theory, loop quantum gravity, and so on). And when we rewind the clock all the way to a hypothetical time "t=0", we get an absurdity: various physical quantities run to infinity- the "initial Big Bang singularity".
Some people, especially in popular science journalism or lay discussions, refer to this as a "beginning of the universe"... but this is dubious, because the fact that our theory predicts an apparently absurd result (i.e. infinite physical quantities, curvature pathology, etc) at precisely the point where we expect it to cease to be applicable (because quantum effects become significant, and GR is not a quantum theory) should tell us we're probably encountering an artifact of a broken theory, not something physically real. To be able to say what, if anything, preceded this state, we would need a successful quantum theory of gravity. Until then, people are free to speculate... but we just don't know either way. But there is no equivalence between the BBT and theistic creation myths- the Big Bang model, at least the part of it that is well-tested and widely-accepted, does not say anything about how the universe began, and unlike various religious myths, is corroborated by a large body of observational evidence.